1. Field of the Invention
The present invention relates to a flat panel display utilizing a thin film organic electro-luminescent element or a liquid crystal and a method of manufacturing the same.
2. Description of the Related Art
There are increasing expectations for flat panel displays such as thin film organic EL displays and liquid crystal displays (LCD's) as displays that replace CRT's (cathode-ray tubes). The main stream of thin film organic EL displays and liquid crystal displays is active matrix type displays that have a TFT (thin film transistor) as a switching element in each pixel to achieve high speed response and lower power consumption. In an active matrix type flat panel display, a TFT must be incorporated in each of pixels that are provided in the form of a matrix on a substrate. In order to provide a greater display screen at a lower cost, pixel TFT's must be formed on a glass substrate that is inexpensive and easy to provide with a large size instead of a quartz glass substrate that is expensive.
TFT's to be formed on a glass substrate include a-Si TFT's having a channel region formed of amorphous silicon (a-Si) and having carrier mobility of about 0.5 cm2/Vs and p-Si TFT's having carrier mobility of about 120 cm2/Vs in which an a-Si layer in a channel region is polycrystallized into poly (polycrystalline) silicon (p-Si) using a low-temperature polysilicon manufacturing process. It is desirable to use p-Si TFT's having high carrier mobility to achieve high speed response and to provide a greater screen or high aperture ratio in an active matrix type flat panel display.
One method for forming p-Si TFT's using a low temperature polysilicon manufacturing process is the excimer laser crystallization method in which a p-Si layer is formed by irradiating an a-Si layer with a pulse-oscillation excimer laser. The excimer laser crystallization method will be described with reference to FIG. 15. FIG. 15 is a perspective view of a glass substrate 100 whose top surface is irradiated with pulse laser light L10 emitted by a XeCl excimer pulse laser 102. A silicon oxide film (SiO2 film) 104 is formed on the top surface of the glass substrate 100 that is placed on an X-Y stage (not shown), and an a-Si layer 106 is formed on the same.
The pulse laser light L10 emitted by the excimer pulse laser 102 deflected by a reflection mirror 108 substantially at right angles to enter a projection optical system 110 at which it is transformed into a divergent pencil of rays having an elongate rectangular projection plane to irradiate the a-Si layer 106. The a-Si layer 106 on the top surface of the glass substrate 100 is sequentially irradiated with the pulse laser light L10 with the X-Y stage that is not shown moved on a step-and-repeat basis, which makes it possible to change the entire a-Si layer 106 into a p-Si layer 112.
For example, the excimer pulse laser 102 emits pulse laser light L10 having 300 pulses/sec, a pulse width PW of 30 ns, a wavelength λ of 308 nm, and power fluctuations of ±10% or less. An area A melt by a single pulse (a surface of the a-Si layer 106 on which the pulse laser light L10 is projected) has a size of 27.5 cm (W)×0.4 mm (L), for example. A relative moving distance per one pulse is 20 μm/pulse in the L-direction (with an overlap of 95%), for example. The scanning speed of the laser light relative to the X-Y stage is 6 mm/sec, for example.
Thus, in the case of excimer laser crystallization, an elongate linear pencil of rays is scanned on the glass substrate 100 having the a-Si layer 106 deposited thereon, each laser pulse being scanned stepwise with an overlap of about 95% in the direction of the minor axis (L-direction in FIG. 15) of the beam spot. The entire a-Si layer 106 on the glass substrate 100 is thus polycrystallized into a p-Si layer 112.
As described above, the use of the excimer laser crystallization method makes it possible to polycrystallize a great width W on a substrate at a time. In the case of an LCD, however, the TFT provided at each pixel is preferably formed in a region that is as small as possible to provide the pixel with a high aperture ratio. In the case of a thin film organic El display, such a region is preferably as small as possible to provide an organic EL element with a great emission area. That is, there are restrictions on a region to be polycrystallized of an a-Si layer on a glass substrate using a low temperature polysilicon manufacturing process. However, since a wide range of an a-Si layer on a substrate is polycrystallized at a time according to the excimer laser crystallization method, a problem arises in that laser light can irradiate even a region where no TFT is to be formed and no crystallization is therefore required and in that polycrystallization suffers from a low throughput because of a low scanning speed of laser light relative to an X-Y stage.